In recent years, progress is being made in research and development of diverse functional elements which involve the use of an organic semiconductor. One typical example of a functional element is an organic EL element. An organic EL element is a current-driven light emitter, and commonly has a pair of electrodes, namely an anode and a cathode, and functional layers layered between the pair of electrodes. The functional layers include a light-emitting layer composed of an organic material. Upon application of voltage across the pair of electrodes, holes injected from the anode to the functional layers recombine with electrons injected from the cathode to the functional layers. The recombination causes the phenomenon of electroluminescence, which involves emission of light. Being self-luminescent, an organic EL element is highly visible. In addition, being completely solid, an organic EL element has excellent impact resistance. Owing to these advantages, more attention is being given to the applications of organic EL elements as a light emitter or a light source for various organic EL display panels and organic EL display apparatuses.
In order to increase the light emission efficiency of an organic EL element, efficient injection of carriers (holes and electrons) from the electrodes to the functional layer is essential. Generally, the provision of injection layers between each of the electrodes and a functional layer is effective in facilitating efficient injection of carriers. This is because an injection layer serves to lower the energy barrier to be overcome in the injection of carriers. An injection layer disposed between a functional layer and the anode is a hole-injection layer composed of an organic material, such as copper phthalocyanine or PEDOT (conductive polymer), or of a metal oxide, such as molybdenum oxide or tungsten oxide. An injection layer disposed between a functional layer and the cathode is an electron injection layer composed of an organic material, such as metal complex or oxadiazole, or of a metal, such as barium.
It has been reported that organic EL elements having a hole injection layer composed of a metal oxide, such as molybdenum oxide or tungsten oxide, exhibit improved hole injection efficiency and longevity (see Patent Literature 1 and Non-Patent Literature 1). It is further reported that the improvement achieved is relevant to the energy level resulting from structures similar to oxygen vacancies of metal oxide on the surface the hole injection layer (see Non-Patent Literature 2).
On the other hand, as organic EL display panels grow in size, it becomes necessary to reduce the resistance of the wiring portion that connects the power source to the electrodes in the organic EL pixels constituting the panel. In particular, in a top emission type active-matrix organic EL display panel, it is necessary to use transparent electrode material, such as ITO or IZO, as the common electrode. As these materials are relatively high resistance, it is desirable to limit their use as a wiring portion.
With respect to this point, for example, Patent Literature 2 discloses a top emission type organic EL element with a wiring portion structured so that the second electrode (common electrode) is connected to auxiliary wiring, thus providing a wiring portion that reduces the use of the relatively high-resistance common electrode. The auxiliary wiring is low-resistance wiring that provides electrons from the power source to the common electrode.
It is desirable to provide the auxiliary wiring in a non-light-emitting cell, so as not to block the light-emitting cell. Furthermore, the auxiliary wiring may be provided either above or below the common electrode in the non-light-emitting cell. A structure in which the auxiliary wiring is provided below can be considered more desirable, as the auxiliary wiring can be formed during the same processes as when forming other components such as the thin-film transistors and pixel electrodes.